Fair Value Measurement - Transfers (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Total debt and equity instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers from level 3 into level 2, assets	$ 472	$ 431	$ 1,245	$ 1,425
Recurring | Derivative payables | Equity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers from level 3 into level 2, liabilities			1,200
Transfers from level 2 into level 3, liabilities			1,000
Recurring | Total debt and equity instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers from level 3 into level 2, assets			1,200
Recurring | Derivative receivables | Equity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers from level 3 into level 2, assets			$ 1,000